Conventional projection lenses used for projecting an image onto a display surface are designed with relatively fast optics. This is particularly true for cinema projection, where traditional film projection lenses may be as fast as ˜f/10.8, and in the emerging technology of digital cinema, lenses are often ˜f/2.5. These low f/# values and correspondingly high angular light are due, in large part, to the large etendue light sources that are used, such as various types of very bright arc lamps and similar light sources, along with the desire to utilize as much of this light as possible.
In the case of digital cinema projection, the image content is provided via pixelated spatial light modulators, such as LCD and LCOS modulators, Digital Micromirror Devices (DMDs), and in particular, the DLP (Digital Light Processor) from Texas Instruments, Inc., Dallas, Tex. Individual pixels of these electronic light modulation devices are modulated on a pixel-addressed basis to impart image data to a transiting light beam. To enable cinema projection, large versions of the devices, with active areas of ˜400-600 mm2 are used, to be compatible and light efficient when used with the large-etendue xenon lamp light sources used for cinema projection. However, we have determined that these large-etendue light sources impact the projector design in various disadvantageous ways, including size and cost of the optical components, thermal load and stress on these components, and the optical imaging performance and image quality provided by the optics. For example, the highly angular light incident transiting the spatial light modulator device, and its associated polarization optics, unfavorably impact the projected image quality, with peak contrast and contrast uniformity deficiencies.
In greater detail, the illumination and projection subsystems of digital projection systems are typically more complex than their equivalents in traditional film-based systems. In particular, in the digital systems, the projection lens systems are often burdened with different and additional requirements compared to the conventional projection optics. As one example, the projection lenses for the digital systems are typically required to provide a long back focal length or working distance, that is, the distance between the last lens surface and the spatial light modulator. Working distances in excess of 2 times the lens focal length are needed in most cases, in order to accommodate a number of optical components used to combine modulated light from the different color paths onto a common optical axis and, depending on the type of spatial light modulator used, to provide polarization, filtering, and other conditioning and guiding of the light. Taken together, the long back focal length and speed requirements (low F#) combine to drive complex lens designs using large elements, as can be well appreciated by those skilled in the optical design arts. As a result, projection lenses used for large venue or digital cinema projection systems are quite expensive, particularly when compared to conventional projection lenses, such as those used in film-based projectors.
As one attempt to reduce this magnitude of this problem, a system, as described in commonly assigned U.S. Pat. No. 6,808,269 entitled “Projection Apparatus Using Spatial Light Modulator” to Cobb, uses imaging relay lenses. Each modulator is imaged by a relay lens to create a real aerial magnified intermediate image near the exit face of a combiner prism. The large numerical aperture (NA) at the modulator plane is reduced, for example by two times, increasing the F# by a corresponding two times. The three-color images are combined through a prism, and then imaged by a common projection lens to the screen. Although the overall system, with the three imaging relays, is increased in complexity, that increased complexity and cost is more than compensated for by the simplicity of the projection lens, which works at a larger F#, without the working distance requirements.
As another approach, the use of visible lasers, having an advantageously small etendue as compared with conventional light sources, offers an opportunity to provide simplified system optics, for example, by enabling projection lenses having similar levels of modest complexity as do the lenses used for film-based projection. In recent years, visible laser light sources have improved in cost, complexity, and performance, thereby becoming more viable for use in projection, including for cinema. Lasers may provide a range of advantages for image projection, including an expanded color gamut, but their small etendue is particularly advantageous for digital systems based on LCDs, DLP, and other types of light modulators, smaller, slower, and cheaper lens elements, with values in the f/8 range or slower may be used, while still providing light of sufficient visible flux for the cinema application, as well as other projection applications. It is noted that lasers also enable other modulator types to be used for projection, such as the Grating Electromechanical (GEMS) modulators, which are linear array devices that utilize diffraction to generate the image data, and which require a small etendue.
Lasers provide many potential substantial advantages for projection systems, including a greatly expanded color gamut, potentially long life sources, and simplified optical designs. However, lasers also introduce speckle, which occurs as result of the coherent interference of localized reflections from the scattering surface of the display screen. Speckle is a high contrast granular noise source that significantly degrades image quality. It is known in the imaging arts that speckle can be reduced in a number of ways, such as by superimposing a number of uncorrelated speckle patterns, or using variations in frequency or polarization. Many of these methods are disclosed in “Speckle Phenomena in Optics: Theory and Applications” by Joseph W. Goodman. As one example of a speckle reduction method pertaining to projection, the display screen is rapidly moved with oscillating motion, generally following a small circle or ellipse about the optical axis. As the screen moves, speckle changes, as localized interactions of the laser light with scattering features are altered by the screen motion. When this oscillating motion is sufficiently fast, speckle visibility is reduced by temporal and spatial averaging, and speckle can become imperceptible to the viewers. Yet another strategy for speckle reduction is to place an optical diffuser at an intermediate image plane internal to the projector, and prior to the projection lens. Oscillation of the diffuser then has the effect of reducing viewer perception of speckle.
A variety of optical diffusers have been used for laser projection speckle reduction, including ground glass, volume, holographic, and lenslet based devices. As one example, in the apparatus disclosed in U.S. Pat. No. 6,747,781 entitled “Method, Apparatus, and Diffuser for Reducing Laser Speckle” to Trisnadi, which uses a diffuser patterned as a Hadamard matrix, in conjunction with a diffractive linear array modulator (GLV) to provide temporal phase variation to an intermediate image of a scanned line of modulated light. This diffuser is constructed of an array of diffusing phase cells, each of which is subdivided into N cell partitions, whose pattern is determined by the Hadamard matrix calculations. An exemplary cell can be 24 μm square and comprise N=64 cell partitions that are 3 μm square. The cell partitions either are an area of the base surface, or a raised, mesa-like area, pi (π) high. If the temporal phase variation provide by the diffuser motion and the cell patterning are appropriate, phase variations in the transiting laser beams are decorrelated, enabling speckle reduction. Specially designed projection and scanning optics are then required in order to project each conditioned line of light onto the display screen. Typically, the projection lens used for such a line-scanned device has an f/# of 2.5. While Trisnadi provides effective reduction of projected speckle, speckle reduction is only one aspect of the design of a laser projection system. Speckle reduction provided by a moving diffuser located at an internal intermediate image plane, that is then imaged to a screen, introduces various further problems, including a reduction image quality (resolution or MTF), light loss from diffusion (scatter or diffraction), and a requirement for faster imaging optics to collect diffused light.
Although many speckle reduction techniques, such as these, exist, there is a continuing need in the art for improved techniques that reduce speckle perception for projected images, while also enabling advantaged designs and system performance from laser projection systems.